1. Field of the Invention
The present invention relates to a converter apparatus that converts AC power supplied from a power source into DC power and supplies the DC power to an inverter to drive a motor, and an inverter apparatus that drives a motor by being supplied with power from such a converter apparatus. More particularly, the invention relates to a method for controlling the DC link voltage appearing across a DC link connected between such a converter apparatus and inverter apparatus, and a converter apparatus and inverter apparatus that can control such a DC link voltage.
2. Description of the Related Art
FIG. 1 is a diagram showing the configuration of a prior art inverter apparatus for controlling the speed of a motor. The inverter apparatus 1 comprises: a converter apparatus 10 for converting AC power supplied from a three-phase commercial power source 2 into DC power; an inverter circuit 20 for converting the DC power into variable-voltage variable-frequency AC power for supply to a motor 3; and a DC link 30 connected between the converter apparatus 10 and the inverter circuit 20 for transmitting the DC power, converted by the converter apparatus 10, to the inverter circuit 20.
The prior art converter apparatus 10 includes a three-phase diode bridge circuit comprising diodes D11 to D16, and converts the AC power supplied from the three-phase AC power source 2 into the DC power by performing full-wave rectification using the bridge circuit.
On the other hand, the inverter circuit 20 comprises a bridge circuit formed by transistors T21 to T26 and diodes D21 to D26 connected in parallel with the respective transistors. Then, an AC voltage signal having a desired voltage and frequency is generated by controlling the ON/OFF operations of the respective transistors T21 to T26 by an inverter control part 21 which is, for example, a transistor PWM control circuit, and the speed of the motor 3 is controlled by supplying the AC voltage signal to the motor 3.
A capacitor C is connected to the DC link 30, and the capacitor C functions to smooth the output of the converter apparatus 10 and to store the regenerative power occurring due to the induced electromotive force produced in the motor 3 when the motor 3 is decelerated or caused to stop. The regenerative power stored in the capacitor C is reused to drive the motor, thus enabling the reuse of regenerative energy (deceleration energy).
The voltage (DC link voltage) applied to the capacitor C changes when the motor is accelerating, running at constant speed, and decelerating. For example, during acceleration, the voltage drops because power is supplied to the motor from the power supply unit; conversely, during deceleration, the voltage rises because energy is fed from the motor back into the power supply unit.
An electrolytic capacitor, for example, is used as the capacitor C. Generally, such a capacitor C has a rated device breakdown voltage, and if a voltage greater than the rated device breakdown voltage is applied to the capacitor, breakdown of the capacitor may occur.
Therefore, in the prior art inverter apparatus 1, a regenerative resistor R is connected in parallel to the capacitor C, as shown in FIG. 1. Then, when the DC link voltage occurring across the DC link 30 exceeds the set voltage, a transistor T3 is turned on, causing the current to flow through the regenerative resistor R and thus reducing the voltage applied to the capacitor C.